FIG. 1 is a cross section showing the layout of an EGR valve (exhaust gas re-circulation valve) as disclosed for example in JP-A-6/147025. In the figure, reference numeral 1 represents the valve housing (valve body), 1a is a first port allowing inflow of engine exhaust gas, lb is a second port allowing inflow of said exhaust gas, 1c is a third port allowing the outflow of exhaust gas inflowing from the first and second ports 1a and 1b, 2 is a valve shaft, 3 and 4 are two valves formed as a unit and secured to the valve shaft 2, 5 is a diaphragm connected to the valve shaft 2, 6 is a set spring tensioning the diaphragm 5 on the valve shaft side 2 (the direction of closure of the valve 3), 7 is a source of negative pressure, 19 is a diaphragm chamber conducting negative pressure from the source of negative pressure 7, 20a is a valve seat secured to the housing 1 by press-fitting or the like and in abutment with one of the valves 3, 20b is a valve seat secured to the housing 1 by press-fitting or the like and in abutment with the other valve 4.
The operation of the invention will now be explained.
Two valves 3, 4 are fixed to respective valve seats 20a, 20b and are in the closed position. When negative pressure is applied to the diaphragm 5, the valve shaft 2 together with the two valves 3, 4 move upwards and put the first and second ports 1a, 1b in the open position and in communication with the third port 1c. When in the open position, engine exhaust gases flow into the valve housing 1 from the first and second ports 1a, 1b and outflow via the third port 1c. The gases may reenter the engine combustion chamber via the intake passage. As a result, it is possible to control increases in engine temperature and combustion of excess oxygen by controlling combustion by the amount of inflammable exhaust gas mixed in the engine combustion chamber even when the engine is on lean burn mode which uses a low mix ratio of air to fuel. It is also possible to control increases in nitrogen oxides (NO.sub.x) which accompany increases in engine temperature and combustion gases which are problems associated with lean burn mode.
However, the following problems are associated with conventional exhaust gas re-circulation valves constructed as above. Although the valve seats 20a, 20b are secured to the valve housing body 1 by press fitting or the like, irregularities easily occur in the process accuracy of the seats or in measurements when the seats are affixed. Such irregularities result in the formation of gaps leading to gas leaks between the valve and the seat when the valve is closed. Furthermore, the high temperature and pressure exhaust gases create overheating and thermal expansion in the valve housing 1, the valve shaft 2 and the valves 3,4. If the rate of the thermal expansion of these respective elements differs, a gap is created between the valve and valve seat and an exhaust gas leak results. Such exhaust gas leaks lead to the problem that, for example, when the engine is idling, extreme decreases in engine revolutions may occur, thus making idling unstable.
It is a purpose of the present invention to solve the above problems by the provision of an exhaust gas re-circulation valve without gaps between the valve and valve seat when the valve is closed, such gaps being caused by irregularities in the process and assembly accuracy of parts such as the valve and the valve seat.
It is a further purpose of the present invention to provide an exhaust gas re-circulation valve which minimizes irregularities in the process and assembly accuracy of parts such as the valve and valve seat as well as prevents the formation of gaps between the valve and valve seat when the valve is closed which are caused by the above irregularities. The exhaust gas re-circulation valve therefore does not allow valve opening when the valve is in the closed position due to the thermal expansion of parts as a result of high temperature exhaust gases.
It is a further purpose of the present invention to provide an exhaust gas re-circulation valve which allows for the simple elimination of gaps formed between the valve and valve seat in the closed valve position due to irregularities in processing or assembling accuracy of parts when the exhaust gas re-circulation valve is assembled.
It is a further purpose of the present invention to provide an exhaust gas re-circulation valve in which the valve seat alone can absorb gaps formed between the valve and the valve seat when in the closed position due to irregularities in the process and assembly accuracy of the parts.
It is a further purpose of the present invention to provide an exhaust gas re-circulation valve in which the valve itself can absorb gaps formed between the valve and valve seat when in the closed position due to irregularities in process or assembly accuracy.
It is a further purpose of the present invention to provide an exhaust re-circulation valve which does not open when in the closed position due to thermal expansion of the valve housing and the valve shaft as a result of high temperature exhaust gas even if for example the rate of thermal expansion of the valve housing and the valve shaft is different.